The invention is represented by a self-threading screw, designed to be inserted in a hole of appropriate size made in a sheet of metal (or other metallic body), which has at the free end of its shank six projections (xe2x80x9cformerxe2x80x9d or xe2x80x9cshapersxe2x80x9d) of prismatic shape designed to thread by rolling the wall of the hole in which the screw itself is inserted.
Self-threading screws (i.e., screws designed to make on the wall of the hole in which they are inserted a thread having a profile complementary to that of the thread present on the shank of the screw) have been known for a long time, are widespread on the market and widely used in technical applications to join together metal sheets or pieces made of a metallic material. With traditional self-threading screws the operation of inserting the screw in the corresponding hole is somewhat troublesome because it is necessary to force the thread of the screw into the wall of the hole to make the required thread; in addition, these screws are not suitable for being used either as adjusting screws or for precision fits, or as means of reversible joining because screwing and re-screwing the screw generally causes a rapid stripping of the thread of the hole. To overcome such drawbacks, various types of self-threading screws have been proposed, which differ from one another according to shape and mode of operation: just to provide a non-limiting example, self-tapping screws are cited (characterized by the presence of longitudinal grooves for discharging the material removed) and screws with three-lobed section and rounded tips. In particular, self-threading screws have been proposed the shank of which presents, at the free end, at least one former of prismatic shape designed to roll the material making up the body to be threaded, so obtaining along the wall of the hole (without any need for further machining) a very resistant complete thread, the turns of which have the fiber intact and slightly hardened by the compression generated by their forming.
In addition, the at least one former of prismatic shape reduces the friction between the thread of the screw and the wall of the hole to be threaded, consequently reducing the effort required for inserting and screwing the screw in the hole; the rolled material, by its relaxation around the screw, increases the coefficient of back-off friction and makes an external thread-internal thread fit that is (rather) precise because the internal thread assumes the profile and the dimensions of the screw.
Available on the market are self-threading screws which have four former of prismatic shape, which have proved able to satisfy a fair share of the technical needs of the market; the use of these screws has proved particularly advantageous for assembling items made of thin sheet metal or other metallic material by means of manual, automatic or semi-automatic systems with high production rates.
Studies and experimental tests conducted by the Applicant have surprisingly made it possible to note that:
the performance (already good) obtained using the above-mentioned known self-threading screws are further improved by increasing the number of former of prismatic shape present at the free end of the shank of the screw from four to six;
a further increase in the number of the former does not further improve the performance obtainable using these self-threading screws but, on the contrary, entails non-negligible constructional and functional problems, due in the first place to the difficulty of locating eight (or more) former along the external circumference of the free end of the shank of the screw.
The subject of the present invention is a self-threading screw for metal comprising six former of prismatic shape set at regular intervals along the external circumference of the free end of the shank of the screw. Preferably but not necessarily, the former have an elongated shape, are parallel to the axis of symmetry of the shank of the screw and are made on the last few turns (from three to five, preferably three) of the thread made on the shank of the screw, adjacent to the free end of the shank.